Photopolymer formulations are known in the prior art. EP 2 154 128 for instance describes a photopolymer formulation containing polyurethane-based matrix polymers, acrylate-based writing monomers, and also photoinitiators. In the cured state, the writing monomers and the photoinitiators form a spatially isotropic distribution embedded in the polyurethane matrix.
For the uses of photopolymer formulations, the crucial role is played by the refractive index modulation Δn produced in the photopolymer by the holographic exposure. In holographic exposure, the interference field of signal light beam and reference light beam (that of two planar waves in the simplest case) is mapped into a refractive index grating by the local photopolymerization of, for example, high-refractive acrylates at loci of high intensity in the interference field. It is the refractive index grating in the photopolymer which is the hologram and which contains all the information in the signal light beam. By illuminating the hologram with only the reference light beam, the signal can then be reconstructed. The strength of the signal thus reconstructed relative to the strength of the incident reference light is called the diffraction efficiency, DE in what follows.
In the simplest case of a hologram resulting from the superposition of two planar waves, the DE is the ratio of the intensity of the light diffracted on reconstruction to the sum total of the intensities of incident reference light and diffracted light. The higher the DE, the greater the efficiency of a hologram with regard to the amount of reference light needed to visualize the signal with a fixed brightness.
High-refractive acrylates are capable of producing refractive index gratings with high amplitude between regions with low refractive index and regions with high refractive index, and hence of enabling holograms with high DE and high Δn in photopolymers. It should be noted here that DE depends on the product of Δn and the photopolymer layer thickness d. The larger the product, the larger the possible DE (for reflection type holograms). The breadth of the angle range at which the hologram is visibly (reconstructed), for example under monochromatic illumination, depends solely on the layer thickness d.
On illumination of the hologram with white light, for example, the breadth of the spectral range which can contribute to the reconstruction of the hologram likewise depends solely on the layer thickness d. The smaller d is, the greater the respective breadths of acceptance. Therefore, if the intention is to produce bright and readily visible holograms, the aim is a high Δn and a low thickness d, so as to maximize DE. This means that, the higher the Δn, the more freedom is achieved to configure the layer thickness d for bright holograms without loss of DE. Therefore, the optimization of Δn is of major importance in the optimization of photopolymer formulations (P. Hariharan, Optical Holography, 2nd Edition, Cambridge University Press, 1996).